1. Field of the Invention
The present invention relates to a method of preparing a dense sintered body formed of zirconia-containing beta alumina (The term "beta alumina" hereinafter includes .beta. and .beta." alumina, and a mixture thereof) for a solid electrolyte. It relates to a method of preparing a dense sintered body being excellent in sodium ion conductivity and useful as a solid electrolytic material for a sodium-sulfur secondary battery and an alkali metal thermo-electric converter.
2. Technical Background and Prior Art
A beta alumina sintered body is produced by a method in which a beta alumina powder material is shaped and sintered. It is also produced by a method in which a mixture of an .alpha.-alumina powder, a sodium compound (e.g., sodium carbonate) and a structure stabilizer (e.g., lithium carbonate) is shaped and sintered. In this method, a process of synthesizing beta alumina by a reaction between an .alpha.-alumina powder and a sodium compound and a sintering process proceed simultaneously.
PCT/EP89/00382 (W089/09755) discloses a method of preparing a ceramic sintered body in which an aluminum powder is mixed with alumina, shaped and sintered. In the method disclosed in this International Publication, the incorporated alumina remains in a sintered body to constitute the sintered body without being altered, and the International Publication does not disclose any reaction in which alumina reacts with a sodium compound to alter itself into beta alumina.
A beta alumina sintered body suitable for a solid electrolyte is required to be dense. Since, however, beta alumina is a substance hard to be sintered, it is difficult to produce a dense sintered body from a beta alumina powder by sintering it. A study is therefore under way to develop a method in which a fine powder of beta alumina is synthesized and sintered. However, beta alumina reacts with water in air to deteriorate, and in particular, a fine powder thereof is highly liable to be decomposed. Therefore, facilities sufficient for storage and handling are required, and it involves difficulty to industrialize the above method.
In the method in which a mixture of an .alpha.-alumina powder, a sodium compound and a structure stabilizer is shaped and sintered, there is a problem in that the reaction does not sufficiently proceed, and an obtained sintered body has residual .alpha.-alumina. As the .alpha.-alumina has no sodium ion conductivity, residual .alpha.-alumina decreases the electrical conductivity, consequently the sintered body is not suitable for a solid electrolyte.
As described above, the techniques for obtaining a beta alumina sintered body suitable for a solid electrolyte have not yet been satisfactory. It is therefore an object of the present invention to provide a method of preparing a dense sintered body or beta alumina suitable for a solid electrolyte.